This invention relates to a bidirectional driving mechanism contained in a feed drive system of a numerically controlled machine tool, for example, for improving servo-characteristics of the machine tool by eliminating backlash and improving mechanical rigidity of the feed drive system.
Generally, it is known that the backlash and the mechanical rigidity of the feed drive system of a numerically controlled machine tool affect adversely the servo-characteristics of the machine tool and cause the lowering of the positioning accuracy of the machine tool. Particularly, in a closed loop type drive system, the backlash and the mechanical rigidity of the drive system cause an adverse effect on the accuracy of the servo-characteristics and likely cause the hunting of the machine tool, which result in the unstable operation of the system.
Since a drive system is constituted by the combination of a driving source and various component elements, the rigidity of the drive system can be improved by increasing the rigidity of respective component elements. Accordingly, the rigidity of the drive system does not vary linearly depending upon the accuracy of the component elements and the error in tightening them and in certain portions of the drive system the rigidity is extremely low.
In order to obviate the above defects of the prior art numerically controlled machine tools or the like; that is, in a double-pinion type drive system in which two pinions are meshing with a rack used for the drive system of the machine tool, the servo-characteristics have been improved by eliminating the backlash and by improving the mechanical rigidity of the drive system by adopting such two systems as (1) a system in which gears of a gear train for driving pinions are helical gears to one of which driving power is continuously applied axially in one direction by means of a hydraulic cylinder or a cup-shaped spring and (2) a system in which two pinions are driven by independent gear trains and a drive source provided with braking means.
However, in the system (1) since the helical gears have surfaces having friction larger than that of a spur gear, the helical gear is not smoothly driven when the direction of rotation is reversed, and furthermore, in the case where a cup-shaped spring is used for urging the helical gear, it is troublesome to adjust a previously applied pressure. In the system (2), an expensive device and control mechanism are required.
FIG. 1 shows a diagrammatic view of a conventional double-pinion drive system of the type wherein power is transmitted to the pinions from one drive source through independent gear trains.
With this drive system, pinions 2 and 3 are engaging with a rack 1 and to these pinions there are secured one ends of shafts 4 and 5 and spur gears 6 and 7 are attached to the other ends of the shafts. The spur gears 6 and 7 are meshing with spur gears 8 and 9 to which shafts 10 and 11 are connected respectively at one ends and to the other ends of these shafts spur gears 12 and 13 are secured. A spur gear 14 is meshing with the spur gears 12 and 13 and secured to one end of a shaft 15 and the other end of the shaft 15 is secured to a bevel gear 16. To this bevel gear 16 another bevel gear 17 is meshed and a drive motor 18 is connected to the bevel gear 17. In this arrangement, when the motor 18 operates, the pinion 2 is driven through the bevel gears 17, 16, and spur gears 14, 12, 8, and 6, and the pinion 3 is also driven through the bevel gears 17, 16, and spur gears 14, 13, 9, and 7.
However, this closed loop type drive system constituted by the rack 1, the pinions 2 and 3, the spur gears 6, 7, 8, 9, 12, 13, and 14 and shafts 4, 5, 10, and 11 produces backlash and possesses a partially inferior mechanical rigidity.